To recover silver from photographic rinse water, a number of alternative methods have been proposed, including the use of ion-exchange resins. Such resins are sufficiently inexpensive and effective to enable their use to reduce silver levels to meet effluent water regulations.
Ion-exchange can be defined as a reversible exchange of ions between solid (resin) and a liquid (water containing ionized salts). Silver in the rinse water is in the ionized form of silver complexes, such as silver thiosulphate compounds, e.g. [Ag(S.sub.2 O.sub.3).sub.2 ].sup.3- and [Ag(S.sub.2 O.sub.3).sub.3 ].sup.5-. Conventionally, to recover the silver, a strong base anion exchange resin is used. In this resin, chloride is the mobile ion and it exchanges with the silver thiosulphate to release a chloride ion while capturing the silver complex, e.g. according to the following scheme: EQU 3[R].sup.+ Cl.sup.- +[Ag(S.sub.2 O.sub.3).sub.2 ].sup.3- .fwdarw.[R].sub.3.sup.+ [Ag(S.sub.2 O.sub.3).sub.2 ].sup.3- +3Cl.sup.-
In a known processing apparatus for processing photographic material, the apparatus comprises a plurality of wet treatment stations including a rinsing station, rinse water from the rinsing station being passed to a silver recovery device for removing silver there-from. The silver recovery device comprises an ion-exchange resin bed and a pump for directing the solution upwardly through the resin bed, the pump being located between the rinsing station and the silver recovery device.
Thus, for example, in U.S. Pat. No. 5,133,846 (Agfa Gevaert N.V. ) there is disclosed a silver recovery device for recovering silver from rinse water from a photographic sheet film processor by means of a fluidized bed of ion-exchange resin.
There exists a phenomena which is referred to herein as "preferential channel formation" which means that liquid does not flow at equal velocities at all points of a cross-sectional area of an ion-exchange bed but, on the contrary, flows at substantially increased velocities at local cross-sectional zones in the form of tiny sub-currents seeking an easy way through the resin bed. The contact time between the liquid and the resin is correspondingly reduced so that the capacity of the device decreases. Also, a fluidized bed allows resin particles at the bottom of the bed to move upwardly and remain, at least for some time, at the top of the bed. Since such resin particles are heavily loaded with silver because of their former presence at the bottom of the bed, silver is likely to become desorbed by water leaving the device. For these reasons, fluidisation of the resin bed is to be avoided.
In a device having a resin bed cross-sectional area of 100 cm.sup.2, a liquid flow rate of 0.5 cm/min, equivalent to a through-put of 50 ml/min is possible. If it is desired to increase the throughput, for example by driving the pump to deliver 500 ml/min, preferential channel formation and fluidization of the resin bed results.
It has been proposed to provide a liquid buffer station between the rinsing station and the resin bed, to assist in the control of liquid flow rate through the silver recovery device, in particular to maintain the linear velocity of rinse water through the resin bed below 0.5 cm/min. While such an arrangement works satisfactorily, a large buffer station places high demands on space.